1. Field of the Invention
The present invention relates to a lathing method and apparatus, a round bar, and a lathed product and, more particularly, to a technique suitable for a so-called CNC automatic lathe (e.g., a guide bush type lathe, and a Swiss-type CNC lathe) for lathing, e.g., a round bar by sequentially feeding the round bar while cantilevering it to be rotatable and fed, unlike lathing in a conventional lathing apparatus in which one end of the round bar is rotatably supported by a lathing chuck and the other end of the round bar is rotatably supported by a tailstock.
2. Description of the Related Art
To normally lathe a round bar called an elongated stock, the intermediate portion of the round bar is rotatably supported by a chuck on the lathing apparatus side, and the other end face of the round bar is centered by a center drill. The machined center is rotatably supported using the tailstock of the lathing apparatus, thereby supporting the two ends of the round bar. After this preparatory process, an apron mounted with a cutting tool is moved along the longitudinal direction of the round bar to cut the round bar. This is general cutting.
On the other hand, a CNC automatic lathe cantilevers a round bar or pipe having a relatively small diameter to be rotatable and fed, and sequentially feeds the round bar with respect to a cutting tool or the like. The above center machining can be omitted, and at the same time parts having small diameters can be centered. For this reasons, the CNC automatic lathe is used properly.
According to this CNC automatic lathe, in a continuous unmanned automatic operation, to improve machining precision of the round bar supported to be rotatable and fed in a cantilevered state, a fixing bush (guide bush) is used to eliminate an off-axis error. For example, a rotary bush type CNC automatic lathe using a rotary bush rotated in interlocking with the round bar is used in practice in a limited field. A lathe of this type is more expensive than a fixing bush type CNC automatic lathe and causes an off-axis error as compared with the fixing bush type CNC automatic lathe. Therefore, machining precision of the rotary bush type CNC automatic lathe is poorer than that of the fixing bush type CNC automatic lathe.
Under the above circumstances, the fixing bush type CNC automatic lathes are currently more popular.
The fixing bush of the fixing bush type CNC automatic lathe will be described with reference to the accompanying drawing. FIG. 1 is a front view of the fixing bush illustrated together with a round bar 1.
Referring to FIG. 1, four expanding slots 3a in a substantially cross shape as shown in FIG. 1 are formed in a fixing bush 3. A hole 3n machined concentric with the outer-diameter portion of the fixing bush 3 is formed in a direction perpendicular to the drawing surface. The hole 3n is formed continuously with the expanding slots 3a.
The hole 3n is formed by a carbide material such as tungsten carbide 3c to sufficiently assure the wear resistance. The hole 3n holds the round bar 1 so that the fixing bush 3 can feed the round bar 1 along the longitudinal direction of the round bar 1. At the same time, the round bar 1 rotated at a high speed in a direction indicated by an arrow in FIG. 1 is sequentially fed to a cutting tool 12 indicated by a broken line, thereby lathing the round bar 1 in the cantilevered state while eliminating an off-axis error.
More specifically, the gap between the hole 3n of the fixing bush 3 and the outer surface of the round bar 1 must be minimized in lathing, and high-precision lathing with a very small off-axis error can be realized.
As described above, lathing using the fixing bush is limited to a metal material or any other industrial material capable of maintaining a sufficient sliding state even if the outer surface of the metal material such as free cutting steel containing a sulfur component, or copper is brought into direct contact with the hole 3n and rotated at a high speed to generate a large amount of heat. In other words, the manufacturer of the fixing bush type CNC automatic lathe does not guarantee use of a material excluding a free cutting metal material such as free cutting steel or copper.
For example, for metal titanium which makes it very difficult to lathe, a dedicated cutting tool whose rake angle is set to about 20.degree. is prepared, the peripheral velocity of the round bar 1 is accurately managed, a cutting liquid set in a state of almost water is sufficiently supplied to cool metal titanium, thereby allowing lathing metal titanium, as is known well.
In such a hard cutting material, however, when the fixing bush 3 is directly used, scoring K occurs, as shown in FIG. 1.
When this scoring K occurs, a workpiece to be cut becomes defective, and scoring K acceleratedly grows. For this reason, in an unmanned automatic operation, the operation must be forcibly interrupted. In the worst case, the apparatus may be undesirably broken, thus posing the decisive problem.
It is said that a hard cutting material cannot be lathed in a fixing bush type CNC automatic lathe.
In order to lathe a stock called an elongated stock into a desired shape, the outer surface of one end portion of the stock is gripped by the chuck of the lathe, and the end face of the stock is cut with an end face tool. Subsequently, the end face of the other end of the stock is centered by a center drill or the like, the chuck is loosened, the stock is fed from the chuck by a machining length, and the machined center hole portion of the stock is rotatably supported by the tailstock of the lathe, thereby supporting the two ends of the stock. After these preparatory operations, an apron mounted with a cutting tool is moved along the longitudinal direction of the stock, thereby cutting the desired portion with the tool. This is general cutting.
To prevent an off-axis error in machining, off-axis error prevention devices each incorporating at least three rotary bearings may be fixed to the lathe in place of the tailstock. The intermediate portion of the stock is rotatably supported to lathe the stock, as needed.
To lathe a metal product, a stock is prepared in advance using a predetermined casting mold so as to obtain a so-called near-net shape similar to the shape of a final product because cutting of a solid material as a specification material into a desired shape results in high cost. Minimum machining and finishing are performed for the above stock using predetermined machine tools, thereby positively reducing the cost. This greatly contributes not only to energy saving in factories and the manufacture of uniform, lightweight products but also to social needs for preservation of terrestrial environment.
FIG. 2 is a front view showing machining on a product having the near-net shape. FIG. 2 exemplifies machining on the shaft mounting hole of a golf club head W as a product. The golf club head W in FIG. 2 is molded to be hollow by a metal casting mold or forging mold (not shown) using metal titanium.
A drilling machine is conventionally used to form the shaft mounting hole in the club head W having the above shape. More specifically, a jig 21 is used such that the machining surface faces upward on a table 20 of the drilling machine. The golf club head W is then fixed in a stationary state. A drill 22 is rotated and moved downward while properly supplying a cutting oil to the machining surface, thereby drilling the head W to a predetermined depth and hence forming a prepared hole. A thread cutting tap 23 having cutting teeth at a desired thread pitch of about M10 is then rotated at a constant speed and moved forward into the prepared hole, thereby performing thread cutting. The tap 23 is then rotated in the reverse direction and moved upward to complete thread cutting.
On the other hand, among the products having near-net shapes, a product which can be directly lathed is set on the chuck of the lathe, thereby lathing the product.
In machining using the above drilling machine, it is difficult to cut a product of a near-net shape made of metal titanium as one of the hard cutting materials which are very difficult to cut. To cope with this, as the drill 22, a dedicated drill having a cutting blade set at a rake angle of about 20.degree. is required. A dedicated tap is also required as the tap 23. As is known well, the hard cutting material can be cut only under the conditions that the peripheral velocity of the spindle of the drilling machine is accurately managed, and the cutting liquid is set in the form of almost water and sufficiently supplied to cool the cutting surface.
In other words, machining is far from continuous cutting represented by NC lathing and must be performed by a machining center or a skilled worker who machines a single item.
Among the products having near-net shapes, a product which can be lathed but is made of metal titanium as one of the hard cutting materials causes scoring due to heat generated between the vibration prevention device and the center hole supported by the tailstock when the product is set on the chuck of the lathe and lathed. When this scoring occurs, the workpiece becomes defective, and scoring acceleratedly grows. For this reason, for example, in an unmanned automatic operation, the operation must be forcibly interrupted. In the worst case, the lathing apparatus is broken, thus posing a decisive problem. Therefore, it is commonly impossible to automatically lathe a hard cutting material.
The present inventors have made extensive studies and examinations on the cause of this scoring K and found the following.
A hard machining material such as the above-mentioned metal titanium, pure titanium, a titanium alloy, and stainless steel has a thermal conductivity of 22 (W/m.multidot.K) or less. This value is smaller than that of a free machining material such as free cutting steel or copper by one or more orders of magnitude.
Frictional heat generated upon rotation between the contact surfaces of the outer surface of the round bar 1 made of, e.g., a titanium bar and the hole 3n of the fixing bush 3, and frictional heat generated upon rotation between the contact surfaces of the outer surface of a stock made of, e.g., metal titanium, the tailstock, and the contact prevention device are gradually accumulated in the round bar 1 (stock). As a result, the round bar 1 thermally expands to increase the diameter of the round bar 1 (stock) and is fused and partially fixed to the hole 3n. This is the cause of the scoring K according to the findings of the present inventors.
Bars and pipes containing hard machining materials are usually wound by antitarnish paper or specialty paper and shipped or managed at the time of shipment or in long-term storage to prevent contamination, damage, and rust. These operations require an extra number of steps to result in high cost.
A large number of lathed products made from round bars and pipes and having solid lubricant layers having predetermined thicknesses on the outer and inner surfaces are commercially available. To obtain these products, predetermined machining operations including lathing are performed, a portion except a prospective solid lubricant film formation portion is masked, and a solid lubricant film is then formed and dried, thereby obtaining a finished product.
For example, Japanese Patent Laid-Open No. 54-137569 discloses a composition for drawing a steel material using the mold release characteristics of TFE (low-molecular weight tetrafluoroethylene) as a fluorine polymer. This composition is effective to assure surface gloss after drawing, but does not positively contribute to formation of the solid lubricant layer. A separate step is required to form a solid lubricant layer having a predetermined thickness on the inner or outer surface of the product in this proposal.
As described above, formation of the solid lubricant layer results in an increase in the number of steps, and extra labor is required in management of semi-finished parts. This increases the cost.